Net Income Per Share (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share [Abstract]
Net income	$ 37.6	$ 30.2	$ 64.7	$ 61.9
Weighted average common shares outstanding (in shares)	70.8	69.5	70.7	69.2
Dilutive effect of stock options, stock appreciation rights and performance share awards, based on the treasury stock method (in shares)	1.6	1.3	1.7	1.3
Assumed conversion of convertible debt, based on the if-converted method (in shares)	0	0	0	0.1
Weighted average shares assuming dilution (in shares)	72.4	70.8	72.4	70.6
Antidilutive options excluded from computation of diluted net income per share (in shares)	0.6	0.9	0.4	0.6